Add descrption of release candidate to roadmap, plus some clarifications.
Add descrption of release candidate to roadmap, plus some clarifications.

timj/scons,timj/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,timj/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,timj/scons,timj/scons,timj/scons,andrewyoung1991/scons,timj/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons,andrewyoung1991/scons